The invention relates to the formation of epitaxial cobalt disilicide layers on silicon substrates using a process based on chemical vapor deposition (CVD) of cobalt.
Due to the demand for faster processing speeds in integrated circuits, the size of features in these circuits has been steadily shrinking. However, as device dimensions are scaled down, increasingly stringent requirements are being placed on the properties and performance of the materials used. For example, the gate length and source/drain junction depth of transistors has been reduced, although the reduction in size causes an increase in resistance. A process termed xe2x80x98self-aligned silicide,xe2x80x99 abbreviated to xe2x80x98salicide,xe2x80x99 is typically used to produce features having acceptable resistance. In a salicide process, a metal is deposited over a metal-oxide-semiconductor (MOS) structure, and reacts with the exposed silicon and polysilicon to form a metal silicide. The unreacted metal is then removed from the MOS structure with a selective etch. In typical processes, the selective etch leaves the silicide over the source/drain regions and on the gates. Since the silicide layer has been formed without using a mask, the process is termed xe2x80x98self-aligned.xe2x80x99 Titanium silicide (TiSi2) is widely used for salicide functions in logic devices. Tungsten silicide (WSi2) has also been used, in particular, for gate applications in dynamic random access memory (DRAM) products. Cobalt disilicide (CoSi2) is a particularly promising material for salicide applications, having properties such as low resistivity, low silicon consumption, and high thermal and chemical stability. Additionally, cobalt disilicide provides the advantage that its properties are independent of feature size and type of silicon dopant. Epitaxial cobalt disilicide is even more highly desirable, given the sharpness of the interface between a cobalt disilicide layer and a silicon substrate, which enables the formation of silicided shallow junctions with low contact resistance and low leakage characteristics. Furthermore, agglomeration is dramatically reduced in the absence of grain boundaries, resulting in superior thermal stability.
An interlayer mediated epitaxy (IME) process is commonly used for growing epitaxial cobalt disilicide. It involves forming an interfacial layer between the silicon substrate and a pure cobalt layer. The process typically uses a titanium interlayer, and is, therefore, called titanium interlayer mediated epitaxy (TIME). Typical TIME processing flow has been described by Wei et al. (U.S. Pat. No. 5,047,367). A titanium layer is initially formed over the silicon substrate by a physical vapor deposition (PVD) method. A cobalt layer is then deposited over the titanium, again by PVD. A first annealing step causes the cobalt and titanium to intermix at about 300xc2x0 C. In a second annealing step, at a higher temperature, intermixing is followed by epitaxial growth of cobalt disilicide, with the titanium being driven to the surface of the resulting cobalt disilicide phase. The titanium surface layer is removed by etching. Other metals, including germanium and tantalum, have also been employed as an interfacial barrier between cobalt and silicon.
The IME and TIME processes are particularly attractive because of compatibility with standard metal-oxide-semiconductor (CMOS) device fabrication flow. However, because the introduction of a reaction barrier interlayer requires additional deposition and etching process steps, there is a need for an improved method for the in situ formation of the interfacial and cobalt layer as one unified growth process. In addition, chemical vapor deposition (CVD) processes have an advantage over so-called xe2x80x98line of sightxe2x80x99 processes such as PVD in the fabrication of semiconductor devices because conformal layers of metals are more easily produced. Therefore, it is desirable that the improved method utilize a CVD process for deposition of the interlayer and cobalt layer.
CVD processes for the deposition of cobalt and cobalt disilicide are known. Ivanova et. al. have disclosed a CVD process for the formation of cobalt films from cobalt tricarbonyl nitrosyl, Co(CO)3NO (J. Electrochem. Soc., 146, 2139-2145 (1999)). The formation of cobalt disilicide was not disclosed. West et al. (U.S. Pat. No. 4,814,294) have described the use of a cobalt source precursor along with silane as a silicon source precursor for the deposition of cobalt disilicide. Dicobalt octacarbonyl, Co2(CO)8 and cobalt tricarbonyl nitrosyl Co(CO)3No are named as suitable cobalt source precursors. The cobalt disilicide deposited was polycrystalline rather than epitaxial; the deposition of pure cobalt films is not disclosed. Rhee et al. have reported a CVD approach for the growth of epitaxial CoSi2 through a two step process which involved, in a first step, the deposition of a Coxe2x80x94C film through the CVD decomposition of the cobalt source dicobalt octacarbonyl, Co2(CO)8, or cyclopentadienyl-cobalt dicarbonyl, C5H5CO(CO)2 (J. Electrochem. Soc., 146, 2720 (1999)). This was followed, in a second step, by an ex-situ thermal annealing step at 800xc2x0 C. to form the epitaxial CoSi2 phase. Unfortunately, the process described has several drawbacks which have prevented commercial acceptance. First, dicobalt octacarbonyl has some serious limitations as a cobalt source precursor. Thermodynamically favorable polymerization and hydrogenation reactions in a CVD chamber compete with the formation of pure cobalt. These reactions include polymerization reactions in the gas phase and reactions with hydrogen yielding highly volatile and extremely unstable hydrocobalt tetracarbonyl compounds. The compound is also known to be unstable during storage, even under vacuum or inert atmosphere. Second, the process also requires a high temperature annealing step (over 800xc2x0 C.) to form the desired CoSi2 epitaxial phase. Finally, an additional etching process step is required to remove a C-rich layer formed over the epitaxial CoSi2 layer.
CVD methods such as atomic layer deposition (ALD) or atomic layer chemical vapor deposition (ALCVD), hereinafter referred to collectively as ALCVD, are attractive methods for the production of device features with sub-tenth micron geometries because of the ability to produce ultrathin, highly conformal layers with atomic-level controllability. In an ALCVD process, a film is grown by repeatedly forming ultrathin layers, with the ultimate thickness of the film being determined by the number of layers deposited. The source precursor is adsorbed on the substrate surface in a self-limiting manner, such that a single monolayer is formed. The precursor is subsequently decomposed to form a single molecular layer of the desired material.
Unfortunately, no ALCVD process for the growth of cobalt or electronic grade cobalt disilicide have been reported. In general, source precursors that have been used for the deposition of cobalt are not amenable to self-limiting adsorption of a monolayer on the substrate required by an ALCVD process. The cobalt source precursors have not shown sufficient stability under ALCVD processing conditions, decomposing prematurely upon contact with the substrate surface, causing the growth of highly contaminated cobalt films. Therefore, there is a need for an ALCVD process for producing ultrathin, conformal pure cobalt and cobalt disilicide films, using cobalt source precursors that are stable under processing conditions and readily adsorb on the substrate to form a monolayer.
It has been unexpectedly found that chemical vapor deposition of pure cobalt films, capping the film with titanium nitride, and annealing the composite structure yields epitaxial cobalt disilicide films. Accordingly, in one aspect, the present invention relates to a process for forming an epitaxial cobalt disilicide layer on a silicon substrate. The process comprises:
(a) vaporizing a cobalt source precursor;
(b) decomposing said cobalt source precursor on the silicon substrate to form on the silicon substrate an ultrathin interfacial layer and a cobalt layer over the ultrathin interfacial layer;
(c) forming, over the cobalt layer, a capping layer comprising at least one of a refractory metal, a refractory metal nitride, a refractory metal carbide, a binary nitride of a refractory metal, a binary carbide of a refractory metal, a ternary nitride of a refractory metal and a ternary carbide of a refractory metal; and
(d) annealing at a temperature sufficiently high and for a period sufficiently long to form the epitaxial cobalt disilicide layer on the silicon substrate.
The cobalt source precursor is selected from cobalt tricarbonyl nitrosyl; cobalt tetracarbonyl iodide, cobalt tetracarbonyl trichlorosilane, carbonyl chloride tris(trimethylphosphine) cobalt, cobalt tricarbonyl-hydrotributylphosphine, acetylene dicobalt hexacarbonyl and acetylene dicobalt pentacarbonyl triethylphosphine, and is preferably cobalt tricarbonyl nitrosyl. The refractory metal is chosen from titanium, tantalum, and tungsten, and is preferably titanium. The refractory metal nitride is preferably titanium nitride, and the capping layer preferably comprises titanium and titanium nitride, and more preferably layer of titanium nitride over a layer of titanium. The capping layer may be formed by physical vapor deposition.
The silicon substrate may be heated to a temperature ranging from 300xc2x0 C. to 600xc2x0 C. in order to decompose the cobalt source precursor on the silicon substrate, and preferably is heated to a temperature of about 390xc2x0 C. Annealing may be performed at a temperature ranging from about 700xc2x0 C. to 1000xc2x0 C., for a period ranging from about 30 seconds to about 90 seconds, and preferably is performed at about 725xc2x0 C., for about 30 seconds. The cobalt layer preferably has a thickness of about 30 nm.
In another aspect, an ALCVD process for producing ultrathin, conformal pure cobalt and cobalt disilicide films, and that uses cobalt source precursors that are stable under processing conditions and readily adsorb on the substrate to form a monolayer has been discovered. Accordingly, the present invention also relates to an ALCVD process for the deposition of a layer comprising cobalt on a surface of a substrate in a deposition chamber. The process comprises:
(a) vaporizing a cobalt source precursor;
(b) pulsing the cobalt source precursor into the deposition chamber;
(c) contacting a surface of the substrate with the cobalt source precursor or a composition derived therefrom;
(d) pulsing an inert gas into the deposition chamber; and
(e) decomposing the cobalt source precursor or composition derived therefrom to form a layer comprising cobalt on the surface of the substrate.
The process may additionally comprise the step of pulsing a reactant gas into the deposition chamber, and/or pulsing an inert gas into the deposition chamber, after pulsing an inert gas into the deposition chamber and before decomposing the cobalt source precursor. The substrate may heated to a temperature ranging from about 300xc2x0 C. to about 600xc2x0 C., and preferably to a temperature of 390xc2x0 C. The steps may be performed repeatedly in sequence to form a plurality of cobalt layers on the substrate.
The cobalt source precursor may be selected from cobalt tricarbonyl nitrosyl, cobalt tetracarbonyl iodide, cobalt tetracarbonyl trichlorosilane, carbonyl chloride tris(trimethylphosphine) cobalt, cobalt tricarbonyl-hydrotributylphosphine, acetylene dicobalt hexacarbonyl and acetylene dicobalt pentacarbonyl triethylphosphine, and is preferably cobalt tricarbonyl nitrosyl. The reactant gas may be hydrogen, and a carrier gas may be pulsed into the deposition chamber with the cobalt source precursor.
In yet another aspect, the invention relates to an ALCVD process for forming an epitaxial cobalt disilicide layer on a silicon substrate which comprises:
(a) vaporizing a cobalt source precursor;
(b) pulsing the cobalt source precursor into the deposition chamber;
(c) contacting a surface of the substrate with the cobalt source precursor or a composition derived therefrom;
(d) pulsing an inert gas into the deposition chamber;
(e) decomposing the cobalt source precursor or composition derived therefrom to form a layer comprising cobalt on the surface of the substrate;
(f) forming a capping layer comprising at least one of a refractory metal, a refractory metal nitride, a refractory metal carbide, a binary nitride of a refractory metal, a binary carbide of a refractory metal, a ternary nitride of a refractory metal and a ternary carbide of a refractory metal; and
(g) annealing at a temperature sufficiently high and for a period sufficiently long to form an epitaxial cobalt disilicide layer.
Annealing may be performed at a temperature ranging from about 700xc2x0 C. to 900xc2x0 C., and preferably at about 725xc2x0 C. The annealing period may range from about 30 seconds to about 180 seconds, and is preferably about 30 seconds. The cobalt layer may have a thickness of about 30 nm.